Within the past few years there has been an increasing concern with the immediate and long-term problems resulting from the ever-increasing pollution of the atmosphere. With this concern has come an awareness at all levels that steps must be taken to halt the increasing pollution and, if at all possible, to reduce the present pollution levels. Accordingly, a substantial amount of effort and money is being spent by business and governmental agencies to develop standards and measures for preventing further significant discharge of pollutants into the atmosphere. Among the pollutants of concern are the various oxides of nitrogen (NO.sub.x) present in the waste gases discharged from many metal refining and chemical plants, such as in nitric acid producing plants, and in the flue gases from power plants generating electricity by the combustion of fossil fuels. A predominant form of nitrogen oxide is nitric oxide (NO). This is a colorless gas which, upon release to the atmosphere, comes into contact with oxygen and can react therewith to form nitrogen dioxide (NO.sub.2) or any of the numerous other oxides of nitrogen. Nitrogen dioxide is a yellow-brown gas known to be toxic to both plant and animal life.
Nitrogen oxides (NO.sub.x) are formed during the combustion of carbonaceous or hydrocarbon fuels in one of two ways. Nitrogen oxides may be formed by a thermal mechanism occurring at elevated temperatures between the nitrogen and oxygen contained in the combustion air (thermal NO.sub.x), or NO.sub.x may result from the oxidation of nitrogen compounds found in the fuel (so-called fuel-bound nitrogen). Heretofore, the principal effect has been directed toward avoiding the formation of thermally formed nitrogen oxide; and various methods are reported in the literature which attempt to inhibit or prevent such formation.
In U.S. Pat. No. 4,050,877, it is proposed to use temperature control to reduce the quantity of thermally formed nitrogen oxides. In accordance with the technique disclosed therein, fuel is burned in a primary combustion chamber with less than the amount of air required for complete combustion so that the formed combustion gases have a high carbon monoxide and hydrocarbon content and the temperature of these formed gases is held below that at which significant quantities of nitrogen oxides would be produced. The combustion gases are then passed through a secondary combustion zone in which more air is injected into the gas stream to oxidize the CO and hydrocarbons to carbon dioxide and water. The secondary burner comprises a plurality of foraminous tubes through which secondary air is emitted. Combustion in the secondary zone also is maintained at a temperature below that at which thermal NO.sub.x will be produced in significant quantities. Similar techniques for minimizing the amount of thermally formed nitrogen oxides are disclosed in U.S. Pat. Nos. 3,837,788; 3,955,909; and 4,013,399.
It also has been suggested that the formation of nitrogen oxides might be avoided by careful mixing of the fuel and air. Thus, numerous methods have been proposed in attempts to obtain substantially uniform mixing during combustion of fuel and air without the formation of nitrogen oxides. For example, it has been suggested that the air and/or fuel be swirled about the combustion chamber or be injected tangentially and the like to achieve uniform mixing and avoid creation of any localized hot spots which could result in the thermal formation of nitrogen oxides. Typical examples of such techniques, in some instances also utilizing temperature control, are found in U.S. Pat. Nos. 3,820,320; 3,826,077; 3,826,079; 4,007,001; and 4,054,028. However, perfect mixing, particularly of air with solid or liquid fuels, while a desirable goal, is difficult if not impossible to achieve. The solid or liquid fuel must first be gasified in order to mix and react with the combustion air. Since the solid or liquid fuel has not begun to gasify or vaporize on first contact with the combustion air, the air/gaseous fuel ratio initially begins at an infinite ratio and decreases to the overall air-fuel ratio as gasification or vaporization proceeds, thereby inevitably creating so-called hot spots. Thus, none of the techniques which rely on uniform mixing have been completely successful in the elimination of nitrogen oxides from the combustion products.
Other patents relating to combinations of temperature control and mixing, with combustion air being introduced in multiple stages, are U.S. Pat. Nos. 4,060,376 and 4,060,378. These patents suggest that the formation of thermal NO.sub.x can be avoided by maintaining the temperature below about 1400.degree. C.; however, it must be appreciated that when withdrawing sufficient heat to maintain the temperature at such low levels, the efficiency of the combustor and the heat transfer coefficients through, for example, a boiler water tube are adversely affected. In addition, emissions of carbon monoxide and unburned carbon and other fuel fragments may also be high.
It also has been suggested in U.S. Pat. No. 4,144,017 that a combination of temperature control and control of the fuel-air ratio for burning the fuel in serially connected furnaces can be an effective means of inhibiting the formation of NO.sub.x. However, this patent, like many of the others, relies upon uniform mixing to avoid forming localized hot spots which result in production of thermal NO.sub.x.
It also has been suggested that certain additives may be introduced into the combustion zone. These additives will decompose in the combustion environment to form reducing materials which will react with and reduce the nitrogen oxides to form nitrogen. The suggested additives include the formates and oxalates of, among others, iron, magnesium, calcium, manganese, and zinc. One obvious disadvantage to this additive process, in addition to the complexity involved, would be the cost of the suggested additives which must be injected into the combustion zone. The reduction of nitric oxide by carbon monoxide over a catalyst consisting of various metal oxides also is known.
It must be appreciated that none of the heretofore discussed methods for reducing the formation of nitrogen oxides specifically addresses reducing the formation of nitrogen oxides from the fuel-bound nitrogen, Rather they are principally directed toward preventing the formation of the thermally formed nitrogen oxides. A need still exists for a method and apparatus for the combustion of fuel which could substantially eliminate nitrogen oxides derived from either source (thermal or fuel-bound) in the combustion products, and which would not rely on achieving perfect uniformity of mixing of fuel and air nor require the injection of expensive additives into the combustion zone nor involve subsequent scrubbing of the combustion products with absorbents.